This project seeks to isolate and characterize the principal lipid and protein components of plasma membranes of embryonic chick lens epithelia and lens fibers, to determine whether the membrane composition changes during lens cell differentiation, and to follow any changes in the rates of synthesis or degradation of membrane components as a function of developmental time. The phospholipid compositions of lens epithelia and lens fibers of six-day-old chick embryos have been determined, and the amount of 32P incorporated into each phospholipid has been measured in vitro and in vivo. The results of these experiments indicate that the most actively metabolized phospholipids in both the lens fibers and lens epithelia are phosphatidic acid and phosphatidylinositol. Measurements of the net synthesis of each phospholipid during a two-hour labeling period in vivo indicate that the net synthesis of phosphatidic acid in the lens fibers is eight to ten times greater that in the lens epithelia, while the net synthesis of other phospholipids is two to three times greater than in the epithelia.